The invention relates to a roller head for a resistance seam welding machine, having a stator, an electrode roller which is rotatably mounted on the stator, a sliding contact current transmission device which is arranged between stator and rotor and is pressed by spring force against at least one of them, and ducts in the stator, the rotor and the sliding contact current transmission device for the passage of a coolant.
Such a roller head is known from U.S. Pat. No. 3,546,655 and from U.S. Pat. No. 3,596,225.
Such roller heads with an internally situated sliding contact current transmission device between stator and rotor could in fact be envisaged as a substitute for the fluid roller heads which have been used up to now, but practice has shown that they are not suitable for this purpose in reality.
Fluid roller heads (CH-A-636 548, U.S. Pat. No. 4,188,523) are used for welding inside narrow can bodies. Between the stator and the rotor there is an annular gap in which there is a fluid metal, for example, quicksilver or a gallium alloy, which serves to transmit current from the stator to the rotor. As such roller heads require careful sealing and constant checking of this sealing, and as the fluid metals used are not particularly good heat conductors, and by comparison with copper, are also not good electrical conductors, and as, moreover, with the use of a gallium alloy, problems may arise from the fact that this metal alloy solidifies at temperatures lying not far below room temperature, such fluid roller heads have already been replaced by a roller head with an externally situated sliding contact current transmission device (EP-B1-0 182 328). This known roller head, which has been developed by the Applicant, does remove the aforementioned problems of fluid roller heads, but requires a reconstruction of the welding arm of the resistance seam welding machine if it is to be incorporated instead of the fluid roller head which has otherwise been used in the case of this machine.
The known sliding contact roller heads hereinbefore mentioned, in view of the overall dimensions, can in fact be used instead of fluid roller heads, but they are inferior to the fluid roller heads in respect of the current transmission capacity and length of life, because they have costly brush constructions as a sliding contact current transmission device or as a constituent of the same. The springs which are necessary for pressing the brushes on, also lead to the following further disadvantage.
In the case of the roller head according to U.S. Pat. No. 3,546,655, there are provided on the stator two horizontal pins, on which brush segments are arranged in the form of a ring and are held together here by 0-shaped rings. The rotor carries convex conductor parts between both brush rings. Between the convex conductor parts and the rings of brushes are provided further conductor parts which are constructed planar on their side facing the rings of brushes and concave on the opposite side. Horizontally arranged compression springs press the brush segments against the further conductor parts and also the latter against the convex conductor parts. With eccentricity or out-of-true running of the rotor, the planar surfaces between the brush rings and the further conductor parts can in fact move vertically relative to each other and the further conductor parts can also perform curved movements on the convex surfaces, but many components are involved in these movements, and considering the high electric currents which have to be transmitted in such roller heads between stator and rotor, all current transmission surfaces moving against each other are basically sources of loss which reduce the load-carrying capacity of the roller heads.
Attempts have been made to remove these problems with the sliding contact roller head according to U.S. Pat. No. 3,596,225, merely by providing on the rotor a cylindrical inner surface with which all the brushes which are provided on the stator are in sliding contact. For this purpose, each brush is pressed against the rotor by a radial compression spring, which is arranged centrally underneath the brush. In this brush construction and also in the brush construction of the sliding contact roller head according to U.S. 3,546,655, the contact pressure between the brushes and the contact surfaces of the rotor coordinated with the latter is disadvantageously influenced by vibrations, for example, through external shocks or through running out-of-true. The current-carrying capacity is accordingly reduced. In other respects, the sliding contact according to U.S. Pat. No. 3,596,225 has a plurality of brushes, all of which represent wear parts and therefore make frequent maintenance necessary.